So the other day I ran over to the LFS for some RO water to do my semi weekly 50% water change. I asked for fresh water. Well, somehow the employee understood that I wanted salt water. I got home, removed around 50% of my water (~5 gallons) and unknowingly replaced it with salt water. I am partially to blame for not checking the water previously but hey I've never had a problem before from that LFS so I trusted the guy. I checked on the tank a few hours later and saw that some of the fish weren't looking so great. I tested the water and saw that the Gh was through the roof. I immediately tasted the extra RO water and found that it was very salty. I rushed to the store for some fresh RO water. I changed as much as I could (~90%) I tested the water again and it was still pretty bad. So I got some more RO water and did another ~90& water change. After that the water was OK. All of the fish recovered except for one oto. Since then my oto has been lazily lounging on the substrate. It won't eat or really even swim around. I'm worried that it might not make it. Any suggestions or ideas would be greatly appreciated. Thanks.

Most catfish cannot tolerate any amount of salt in the water. I would do a 100% water change. Don't scrub down anything or clean the filter, but take as much of the water out as possible (leave just enough for the fish to swim around in) and replace the rest.

I have done 2 ~90-100% water changes. I use RO water because where I live my tap water is very hard and pretty basic. The fish I have prefer soft slightly acidic water. I add alkaline buffer to my RO water to bring the pH up just a bit and prevent pH crashes due to DIY CO2

The only thing that will really help him now is fresh, clean water. He may have been damaged beyond repair... the only thing to do is wait it out and make sure the water is changed regularly.

I'm concerned with the amount of "fiddling" you're doing with your water chemistry. It's said on here multiple times "stable pH is better than an unstable pH," even if you're gearing for the "perfect" pH.

I'm not really "fiddling" that much. I add one thing to my water before I add it to my tank. Its always the same amount too. The pH stays at a very constant 6.8.

As for why I don't mix RO and tap water. I tried that and found it hard to get equal proportions every time I mixed the water. Through personal experience I found it much easier to just add a little alkaline buffer to my RO water before I add it to my tank. So far it has worked out really well.